A cold cathode tube has conventionally been used as a backlight of a liquid crystal display device. The cold cathode tube has a longer operating life than a hot cathode tube, so that it is suitably used for a backlight of a liquid crystal display device which can be used over a long period of time in various fields such as a television, a personal computer, a cellular phone and a pinball machine. The cold cathode tube generally takes a configuration in which a pair of electrodes for cold cathode tube formed by coating surfaces of high melting point metal electrodes made of Ni, Mo or the like by an electron emissive material (emitter material) such as LaB6 and BaAl2O4 are placed opposite to each other inside a glass bulb (glass tube) (refer to Reference 1). Generally, the electrode for cathode tube has a bottomed cylindrical shape.
The conventional bottomed cylindrical electrode is manufactured by performing punching on a plate material (high melting point metal plate material) formed by hot rolling (or cold rolling) an ingot manufactured by a melting method or a sintered body manufactured by a powder metallurgy method. The processing is also called as a drawing when manufacturing a bottomed cylindrical member. For mass-producing the electrode for cold cathode tube, a complicated punching device such as a transfer press and a progressive press is used.
In order to apply the punching, it is required to perform preprocessing such as rolling on the high melting point metal plate material so that its thickness is sufficiently reduced. Further, when the cylindrical electrode is manufactured by the punching, a punching waste is inevitably generated, so that it is difficult to fully use 100% of the plate material (raw material). Tentatively, when the punching waste is reused, there is a need to apply the melting method to manufacture the plate material again. Either of the above becomes a factor for increasing a manufacturing cost for the electrode for cold cathode tube.
As described above, the manufacturing method of the cylindrical electrode applying the punching includes a lot of factors that increase the manufacturing cost, so that it has been difficult to manufacture the cylindrical electrode at a low cost. Further, the high melting point metal plate material manufactured by the melting method or the powder metallurgy method has a relative density of substantially 99% or more and thus has no pore on a surface thereof, so that a surface area thereof is small, which is a drawback. For this reason, when the electron emissive material is applied to the surface, it is only possible to obtain an applied area equal to the surface area.
An electrode for cold cathode tube formed of a sintered body of high melting point metal powder such as W is disclosed in Reference 2. Since this electrode uses the sintered body, it can be manufactured at a lower cost than the electrode applying the punching. However, the shape of the electrode is a cylindrical body with no bottom portion (hollow body), which creates a drawback that a surface area of the electrode is insufficient. When the surface area is insufficient, it is not possible to sufficiently obtain a hollow cathode effect. A partition is provided to eliminate the insufficiency of the surface area in Reference 2, but, it is difficult to manufacture, with such a shape, a small-sized electrode having a diameter of 3 mm or less.
A cold cathode tube is configured by providing a phosphor layer which is excited by ultraviolet light in an inner surface of a glass tube, and by sealing minute amounts of mercury and rare gas in the tube. When a voltage is applied to electrodes provided in both ends of the glass tube, the mercury is evaporated, resulting in emission of ultraviolet light, and the ultraviolet light makes the phosphor layer emit light. When the cold cathode tube is used over a long period of time, a sputtering phenomenon of the electron emissive material (emitter material) and an electrode material is occurred. The mercury inside the tube is taken into a sputtered layer formed by the sputtering phenomenon, resulting that a light emission efficiency and an operating life of the cold cathode tube are decreased.
Reference 3 discloses that a convex portion is provided inside an electrode for cold cathode tube to increase a surface area for suppressing the sputtering phenomenon. The sputtering phenomenon is suppressed by increasing the surface area and the amount of coating of the electron emissive material. However, the electrode disclosed in Reference 3 is not the bottomed one, so that there is a limit in increasing the surface area. Particularly, in a thin electrode whose diameter is 3 mm or less (hollow cylindrical electrode), even if the convex portion is provided therein, there is a limit in increasing the surface area.
In order to improve the above-stated points, Reference 4 and Reference 5 disclose an electrode for cold cathode tube made of a sintered body of W, Nb, Ta, Mo or the like. By using the electrode for cold cathode tube made of the sintered body of W, Nb, Ta, Mo or the like, it is possible to reduce costs and obtain an effect of improvement in the consumption amount of mercury and the like. However, an inner surface of the electrode for cold cathode tube disclosed in Reference 4 and Reference 5 has a cross section of a horseshoe shape in which a bottom portion and an opening portion have the same shape, or of a V shape (or U shape) in which the cross-sectional shape is gradually enlarged from the bottom portion toward the opening portion.
The conventional electrode for cold cathode tube has a problem that it cannot sufficiently suppress the sputtering phenomenon in which an electrode material scatters when ions collide with the electrode during lighting, and deposits on an inside wall of the lamp (cold cathode tube). When the sputtering phenomenon occurs, the mercury inside the cold cathode tube is taken up, and thus is not usable for discharge. Accordingly, when lighting for a long period of time, almost all of the mercury inside the tube is taken into a sputtered layer, which extremely lowers brightness of the lamp, resulting that the lamp reaches the end of its operating life. Therefore, if the sputtering phenomenon can be suppressed, the consumption of mercury can be reduced, which enables to realize a longer operating life even with the same amount of the sealed mercury.
Regarding the above-stated point, the conventional electrode for cold cathode tube having a cross section of a horseshoe shape or a V (U) shape cannot sufficiently suppress the sputtering phenomenon. Further, the electrode for cold cathode tube is used in a state in which a lead terminal is joined thereto. The electrode for cold cathode tube (sintered body electrode) disclosed in Reference 4 and Reference 5 has a thicker wall thickness at the bottom portion side, so that it is inferior in weldability of the lead terminal, which is a drawback.
Reference 1: JP-A 62-229652
Reference 2: JP-A 04-272109
Reference 3: JP-A 2002-025499
Reference 4: JP-A 2004-178875
Reference 5: JP-A 2004-192874